This invention relates to analytical sensors for the detection of bioanalytes in a small volume sample.
Analytical sensors are useful in chemistry and medicine to determine the presence and concentration of a biological analyte. Such sensors are needed, for example, to monitor glucose in diabetic patients and lactate during critical care events.
Currently available technology measures bioanalytes in relatively large sample volumes, e.g., generally requiring 3 microliters or more of blood or other biological fluid. These fluid samples are obtained from a patient, for example, using a needle and syringe, or by lancing a portion of the skin such as the fingertip and xe2x80x9cmilkingxe2x80x9d the area to obtain a useful sample volume. These procedures are inconvenient for the patient, and often painful, particularly when frequent samples are required. Less painful methods for obtaining a sample are known such as lancing the arm or thigh, which have a lower nerve ending density. However, lancing the body in the preferred regions typically produces submicroliter samples of blood, because these regions are not heavily supplied with near-surface capillary vessels.
It would therefore be desirable and very useful to develop a relatively painless, easy to use blood analyte sensor, capable of performing an accurate and sensitive analysis of the concentration of analytes in a small volume of sample.
Sensors capable of electrochemically measuring an analyte in a sample are known in the art. Some sensors known in the art use at least two electrodes and may contain a redox mediator to aid in the electrochemical reaction. However, the use of an electrochemical sensor for measuring analyte in a small volume introduces error into the measurements. One type of inaccuracy arises from the use of a diffusible redox mediator. Because the electrodes are so close together in a small volume sensor, diffusible redox mediator may shuttle between the working and counter electrode and add to the signal measured for analyte. Another source of inaccuracy in a small volume sensor is the difficulty in determining the volume of the small sample or in determining whether the sample chamber is filled. It would therefore be desirable to develop a small volume electrochemical sensor capable of decreasing the errors that arise from the size of the sensor and the sample.
The sensors of the present invention provide a method for the detection and quantification of an analyte in submicroliter samples. In general, the invention includes a method and sensor for analysis of an analyte in a small volume of sample by, for example, coulometry, amperometry and/or potentiometry. A sensor of the invention utilizes a non-leachable or diffusible redox mediator. The sensor also includes a sample chamber to hold the sample in electrolytic contact with the working electrode. In many instances, the sensor also contains a non-leachable or diffusible second electron transfer agent.
In a preferred embodiment, the working electrode faces a counter electrode, forming a measurement zone within the sample chamber, between the two electrodes, that is sized to contain no more than about 1 xcexcL of sample, preferably no more than about 0.5 xcexcL, more preferably no more than about 0.25 xcexcL, and most preferably no more than about 0.1 xcexcL of sample. A sorbent material is optionally positioned in the sample chamber and measurement zone to reduce the volume of sample needed to fill the sample chamber and measurement zone.
In one embodiment of the invention, a biosensor is provided which combines coulometric electrochemical sensing with a non-leachable or diffusible redox mediator to accurately and efficiently measure a bioanalyte in a submicroliter volume of sample. The preferred sensor includes an electrode, a non-leachable or diffusible redox mediator on the electrode, a sample chamber for holding the sample in electrical contact with the electrode and, preferably, sorbent material disposed within the sample chamber to reduce the volume of the chamber. The sample chamber, together with any sorbent material, is sized to provide for analysis of a sample volume that is typically no more than about 1 xcexcL, preferably no more than about 0.5 xcexcL, more preferably no more than about 0.25 xcexcL, and most preferably no more than about 0.1 xcexcL. In some instances, the sensor also contains a non-leachable or diffusible second electron transfer agent.
One embodiment of the invention includes a method for determining the concentration of an analyte in a sample by, first, contacting the sample with an electrochemical sensor and then determining the concentration of the analyte. The electrochemical sensor includes a facing electrode pair with a working electrode and a counter electrode and a sample chamber, including a measurement zone, positioned between the two electrodes. The measurement zone is sized to contain no more than about 1 xcexcL of sample.
The invention also includes an electrochemical sensor with two or more facing electrode pairs. Each electrode pair has a working electrode, a counter electrode, and a measurement zone between the two electrodes, the measurement zone being sized to hold no more than about 1 xcexcL of sample. In addition, the sensor also includes a non-leachable redox mediator on the working electrode of at least one of the electrode pairs or a diffusible redox mediator on a surface in the sample chamber or in the sample.
One aspect of the invention is a method of determining the concentration of an analyte in a sample by contacting the sample with an electrochemical sensor and determining the concentration of the analyte by coulometry. The electrochemical sensor includes an electrode pair with a working electrode and a counter electrode. The sensor also includes a sample chamber for holding a sample in electrolytic contact with the working electrode. Within the sample chamber is sorbent material to reduce the volume sample needed to fill the sample chamber so that the sample chamber is sized to contain no more than about 1 xcexcL of sample. The sample chamber also contains a non-leachable or diffusible redox mediator and optionally contains a non-leachable or diffusible second electron transfer agent.
The sensors may also include a fill indicator, such as an indicator electrode or a second electrode pair, that can be used to determine when the measurement zone or sample chamber has been filled. An indicator electrode or a second electrode pair may also be used to increase accuracy of the measurement of analyte concentration. The sensors may also include a heating element to heat the measurement zone or sample chamber to increase the rate of oxidation or reduction of the analyte.
Sensors can be configured for side-filling or tip-filling. In addition, in some embodiments, the sensor may be part of an integrated sample acquisition and analyte measurement device. The integrated sample acquisition and analyte measurement device may include the sensor and a skin piercing member, so that the device can be used to pierce the skin of a user to cause flow of a fluid sample, such as blood, that can then be collected by the sensor. In at least some embodiments, the fluid sample can be collected without moving the integrated sample acquisition and analyte measurement device.
One method of forming a sensor, as described above, includes forming at least one working electrode on a first substrate and forming at least one counter or counter/reference electrode on a second substrate. A spacer layer is disposed on either the first or second substrates. The spacer layer defines a channel into which a sample can be drawn and held when the sensor is completed. A redox mediator and/or second electron transfer agent are disposed on the first or second substrate in a region that will be exposed within the channel when the sensor is completed. The first and second substrates are then brought together and spaced apart by the spacer layer with the channel providing access to the at least one working electrode and the at least one counter or counter/reference electrode. In some embodiments, the first and second substrates are portions of a single sheet or continuous web of material.
These and various other features which characterize the invention are pointed out with particularity in the attached claims. For a better understanding of the invention, its advantages, and objectives obtained by its use, reference should be made to the drawings and to the accompanying description, in which there is illustrated and described preferred embodiments of the invention.